Presently, there is much activity in the development of spread spectrum communications systems in both the commercial and military arenas. A spread spectrum communication system is a system in which a plurality of communication units (radios) communicate with each other over a wide band of frequencies within a single communications channel. As a result, no one frequency is dedicated to any one communication network. This frees-up air-space so that a greater number of systems can use the limited number of available frequencies for communication over the air. Consequently, spread spectrum systems provide a more economical solution for over-the-air multiple access communications.
One technique for implementing spread spectrum communications is frequency hopping. In a frequency hopping system, the carrier frequency shifts from frequency to frequency in a predetermined pseudo-random pattern throughout the spectrum of the communication channel at predetermined times, based on the network clock. The network clock is established when the local clocks of all the units communicating on the channel are substantially synchronized to the same time. Without this local clock synchronization or network time, the units communicating on the channel will not hop to next frequency at the same time, and thus will lose communication with each other.
Essentially, frequency hopping acts as a time-frequency coding technique that provides a high degree of protection from frequency jamming as well as protection from eavesdropping on the communications channel. As a result, its success is directly dependant on the accuracy of the network timing. Since the network time is based on the local clocks of all the units communicating on the channel, it is not dependant or linked to the actual time of day.
One method of establishing and maintaining an accurate network time between the units communicating on the channel is disclosed in U.S. Pat No. 5,121,408, entitled "Synchronization For Entry To A Network In A Frequency Hopping Communication System," issued Jun. 9, 1992, to Cai et al, and incorporated herein by reference. Cai et al discloses a synchronization arrangement in which each unit on the network is initially synchronized to the same time or initial network time. As communications take place, each unit continually tracks its local clock deviation from that of the network time, making any corrections necessary to maintain synchronicity between its local clock and the network clock. As a result, each time a unit begins to transmit on the network all the member units must resynchronize with each other.
Consequently, this synchronization process, which is inherent to all present day FH communications systems, significantly limits the rate of interactive communications on the channel. Every time the channel is invoked, the transmitter and receiver must be synchronized, taking away from the available time for transmitting the actual data. Moreover, units intending to use the channel will incur a channel access delay to insure that the channel is free and to avoid colliding with other units communicating or waiting to communicate on the channel. As a result, these synchronization and channel access delays occupy valuable channel time and become a limiting factor in channel throughput.
One system in which this re-synchronization protocol is a great burden is the United States Army's Single Channel Ground and Airborne Radio System (SINCGARS). In the SINCGARS system, the transmitter sends a re-synchronizing signal over the channel before each transmission. Since typical communications on the SINCGARS system involves many data packets having a short duration compared to the re-synchronization time, the re-synchronizing overhead can monopolize a large percentage of the air time. As a result, the SINCGARS throughput, and thus communications are severely burdened.